With a single pour for a concrete path, slab or pavement, joints are installed at intervals in an effort to have the joints crack and thereby accommodate subsequent concrete shrinkage/expansion either side of the joint. One way in which such joints are provided is by forming crack promoting grooves in the exposed top surface of the concrete. These grooves can be formed in the concrete with tools or inserts as part of the concrete finishing process or they can be formed by concrete saw-cutting after the concrete finishing process. These grooves provide lines of weakness along which cracks can develop during the life of the concrete path, slab or pavement. Another way in which such joints are provided is by casting in a crack promoting insert during concrete placement. These crack promoting inserts positioned prior to concrete placement provide lines of weakness along which cracks can develop during the life of the concrete path, slab or pavement. Crack promoting inserts can be used in a joint in combination with a crack promoting groove. Usually, in such cases, the insert is immediately below the groove although the insert may be below and to one side of the groove if desired.
The time taken for a crack promoting groove or insert alone or in combination to create a crack depends on a range of variables including friction developed between the underside of the concrete path, slab or pavement and the supporting sub-grade, rate of concrete tensile strength development in the concrete, ambient temperature changes, rate of drying shrinkage of the concrete etc. Some crack promoting grooves and inserts never create a crack. Ideally all crack promoting grooves and inserts alone or in combination would create cracks immediately after completion of construction of the concrete path, slab or pavement. Then each crack created by a groove or insert would accommodate only the concrete shrinkage/expansion either side of that groove or insert.
In practice, both crack promoting grooves and inserts alone or in combination create cracks at varying times after completion of construction of the concrete path, slab or pavement and sometimes no crack is created. When cracks are created at varying times the crack promoting groove or insert that creates a crack first must accommodate all the concrete shrinkage/expansion of the concrete until another crack promoting groove or insert creates a crack. This results in joints with crack promoting grooves or inserts that create cracks early opening wider than those with crack promoting grooves or inserts that create cracks later. Cracked joints that open too wide are undesirable as they typically perform poorly during the life of the concrete path, slab or pavement.
Crack promoting grooves that are formed by concrete saw-cutting are often cut deep in an effort to promote cracking. This is expensive and the deep saw-cut reduces the ability of the cracked joint to transfer shear loads across the joint using aggregate interlock of the cracked concrete on each side of the cracked joint.